Because the Jews didn't know Jesus, they didn't
and couldn't know the Father. The awful result
being that they would die in their sins:

"Then said Jesus again unto them, I go my way,
and ye shall seek me, and shall die in your sins:
whither I go, ye cannot come" John 8:21

As humans, we know nothing of our beginning or
our ending beyond what has been revealed to us.
Only Jesus has this knowledge, and so is qualified
to be a witness.

He was from above and they were from beneath.
If they didn't believe in Him, they would die in
their sins. That really goes for everyone.